Shower base



FIG. 1 is a perspective view of a shower base showing our new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a side view thereof; and,

FIG. 4 is a front elevational view thereof.

The broken line showing of a plumbing drain is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a shower base, as shown and described. 